Nowadays, following the development of digital products, market demand of small sized optical lenses, such as pick up lenses, digital camera lenses, cell phone camera lenses, has extremely increased. An optical axis of a lens is the axis passing through two centers of curvature of lens surfaces. A mechanical axis passes through the physical center of the lens. If the optical axis of a lens is parallel to but not coincident with the mechanical axis of the lens, a decenter error exists. The decenter error negatively impacts the optical performance of the lens. Therefore, detection of decenter error of a lens is an important issue in the manufacturing of optical lenses.
Conventional methods used for measuring the decenter error of a lens include mechanical measurement and optical measurement. The optical measurement is more precise and popular than the mechanical measurement. Transmissive type and reflective type measurements are commonly utilized in optical measurement. The transmissive type measurement is achieved by projecting a light beam in a cross-line configuration from a collimator focused on a focal plane of the lens. An additional autocollimator is needed for emitting a beam parallel to the focal plane of the lens. The image projected onto the focal plane of the lens is observed through an eyepiece of an autocollimator. When a decenter error is present, the observed image describes a circle and the lens is rotated around a reference axis. A diameter of this circle is proportional with an amount of the decenter error. As a result, the radius of the circle can be taken as the amount of decenter error.
As lens technology develops, optical lenses formed by glass molding and injection forming become more popular. However, with these methods, burs and mouse bites may be formed along the annular sidewall of the lenses. As a result, measurement of decenter error may be less accurate.
In view of the above, there is a need to provide an apparatus for measuring decenter error of a lens.